Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 11 4 2007 10.5194/hess-11-1405-2007 http://www.hydrol-earth-syst-sci.net/11/1405/2007/ http://www.hydrol-earth-syst-sci.net/11/1405/2007/hess-11-1405-2007.html http://www.hydrol-earth-syst-sci.net/11/1405/2007/hess-11-1405-2007.pdf 1405 1416 2007-07-06 A conceptual investigation of process controls upon flood frequency: role of thresholds I. Struthers M. Sivapalan sivapala@uiuc.edu School of Environmental Systems Engineering, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia Centre for Water Research, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia now at: Department of Geography and Department of Civil & Environmental Engineering University of Illinois at Urbana-Champaign, Urbana, Illinois, IL 61801, USA Traditional statistical approaches to flood frequency inherently assume homogeneity and stationarity in the flood generation process. This study illustrates the impact of heterogeneity associated with threshold non-linearities in the storage-discharge relationship associated with the rainfall-runoff process upon flood frequency behaviour. For a simplified, non-threshold (i.e. homogeneous) scenario, flood frequency can be characterised in terms of rainfall frequency, the characteristic response time of the catchment, and storm intermittency, modified by the relative strength of evaporation. The flood frequency curve is then a consistent transformation of the rainfall frequency curve, and could be readily described by traditional statistical methods. The introduction of storage thresholds, namely a field capacity storage and a catchment storage capacity, however, results in different flood frequency "regions" associated with distinctly different rainfall-runoff response behaviour and different process controls. The return period associated with the transition between these regions is directly related to the frequency of threshold exceedence. 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